User blog:Samuli.seppanen/Hunt for the 65 cm power stroke
In last few weeks I've been making various small enhancements to the cheiroballistra. I made the triggering mechanism much more robust by (temporarily) using a small nut and bolt as the trigger axle. In the final version I'll probably use a small rivet or maybe a strong nail. I also replaced the nut used as the claw axle with a more rigid axle made from uniformly round steel. My first crescent-shaped piece was way too narrow and rough for high-power tests - my abs were actually sore for several days after my previous shooting session last weekend. So, I made a new crescent-shaped piece by laminating several thin sheets of 10 cm wide ash together. The process was way too complex for my taste, and it included making a bending form, steaming, bending and finally gluing the wood layers together. Additionally I had to make a special piece of wood - an adapter of a kind - for the rectangular, wooden tenon that is sunk into the crescent-shaped piece and the end of the case. All in all, the process was overly complicated, and I even had resorted to using screws to make things a bit simpler. Still, I'm willing to accept this new component as an intermediate solution used for testing while my perfectly crooked piece of spruce is still drying in the attic. The ballista looks like this currently: The pulling handle helps the draw significantly, but the handgrip part has to be reinforced significantly to allow care-free full-power draws. If pull the handle without using any stomach pressure, I can fairly easily pull the arms to perpendicular to the case (with current pretension level). If I use stomach pressure alone, I can push the arms slightly past the perpendicular with reasonable amount of strain. Combining both techniques of course produces the best results. So far I have not managed to fully draw the cheiroballistra because of excessive stacking, i.e. rapid increase of draw weight towards the end of the draw. Below are two sets of graphs - first start and end position of the arms followed by the associated force-draw curve. The horizontal axis in the force-draw curve graphs below represent the draw length in centimeters, and these graphs clearly show both the stacking point, as well as the incredibly light initial draw. The very light draw at the beginning means extremely high bowstring acceleration at the end of the shot, which should result in very good efficiency levels even with light bolts. The graphs were drawn by an Octave program written by my good friend Boris who is more learned in the arts of mathematics and physics than I am. The first three pictures show an arm configuration where the arms are initially in parallel to the groove in the slider: As can be seen, the draw weight starts rising sharply after 50 centimeters. If we lengthen the bowstring significantly so that arms point 30 degrees outwards, the rise of draw weight starts at about 60 centimeters: Of course these models are built on the premise that the bowstring does not stretch. In practice, stretch of the bowstring could have a fairly significant impact on draw length. For example, if the bowstring is 80.4 cm long as in the latter case above, we get * 2% stretch: 1.97 cm elongation * 5% stretch: 4.02 cm elongation The actual percentage will depend on the thickness of the bowstring and the final draw weight. The model also assumes that the "claw" is a single point of zero width. My claw is about 2.1 cm wide, and that amount of bowstring is "wasted" as far as draw length is concerned. In any case it seems that I have to file down the curves in the field-frame bars to allow longer bowstring and a longer draw. Any other solution would not make mechanical sense, would violate the text, or both: * Moving the projecting block below the case to a more "suitable" place is out of the question. Although there is an error in the text, Prou's (1877: 120-121) corrected it elegantly with a minimal amount of assumptions. For details on the other suggested corrections see Iriarte (2000: 48). * Moving the little ladder to the other side of the projecting block makes no sense. If it was done, the little ladder's T-clamps would have to made significantly heavier and the projecting block would no longer serve it current useful and logical purpose. * Placing the little ladder inside the projecting block makes no sense. * Lengthening the hooks in the arms enough to make a difference would make them much more fragile or much heavier and slower, as discussed here. If the Romans had wanted to make the arms longer, they would have lengthened the bars and cones to keep maintain the integrity of the arms. * Moving the butt-ends of the cones towards the center of the spring bundle is not possible without risking the spring cords from slipping over them. The only way to accomplish this would be to use spiral pretensioning. However, a heavy price would have to be paid for doing that. * Moving the field-frames farther away from each other is not possible to the extent necessary. The length of the little arch pretty much determines the maximum distance of the field-frames, and my current reconstruction is at that very limit already. On top of this in my working reconstruction the location of the little ladder (or more correctly, the bowstring) looks very reasonable, with the slider extending about 16 centimeters beyond the bowstring. This should be enough to stabilize a short bolt after it has lost contact with the bowstring should the arms strike slightly out of balance. The hunt for the missing 5-7 centimeters continues... Category:Blog posts Category:Backup